PARIS, April 13 - An arson attempt left very minor damage at the Paris building where French presidential candidate Marine Le Pen has her headquarters, a police source said, and Le Pen accused leftist groups of the attack.The police source said the ground floor of the central Paris building was targeted and graffiti mentioning Le Pen's National Front was found nearby. The source said the damage - to a door and a doormat, according to news agency AFP - was likely to be the result of a criminal act and not an accident.The party's premises are higher up in the building.Interior Minister Matthias Fekl condemned the attack."These are unacceptable acts, the democratic debate must take place in the ballot box," Fekl told RTL radio, without giving any details about the attack itself."We have been in touch with the National Front candidate's team since last night and will see with them if it is necessary to strengthen security procedures."Le Pen told France 2 television she believed a leftist group was responsible, but she gave no detail and did not say why she believed one such group was responsible."I assume this is due to a small leftist group," she said. "These groups act in total impunity," she added, saying that the government should dissolve them.